Whenever I read about terrorist activities being perpetrated in the name of religion and specifically when it comes from Islam, I get intrigued whether a presence of a religious symbol that would have depicted peace and love would have been of any value for people who really can't understand abstract thoughts.
Abstract thinking such as internalizing the subtleties of universal brotherhood are really not easy concepts for a raw mind. Its very very difficult to go beyond the confines of the structures and historical events that impose a certain kind of perspective on the minds of people. While just emulating the symbol in some form can help in a very simple way.
Certain religious symbols inspire a certain kind of virtue. Eg. the deep stillness and highest form of sacrifice in Jesus's image encourages one atleast not to ignore these values if not emulate them on a daily basis. Similarly, Buddha's image in meditation inspires the need for internal deep dive. In Hinduism, Lord Krishna's image goes to a large extent in encouraging the virtue of dharma and prosperity- (though the latter ideal is what is generally seen to be understood by the simple minded and the former conveniently ignored).
At times I would say that if the image doesn't inspire the actual act it surely challenges the contrarion line of thinking. For example not all Christians may believe in sacrifice and surely not in living a life of non-materialism but the symbol would surely contest any feeling on the contrarion side, of say killing others in the name of religion. Similar would be the case with symbols of other religions. Which means that just because Hinduism has symbols doesn't imply that all Hindus embrace love and peace. But surely it mitigates and challenges people who want to perpetrate violence in the name of religion.
While when we look at Islam, we find that being a monotheistic religion, it differs from Christianity in not having any symbol of worship. I would surely want to understand the reason of this design but I wonder if some symbol signifying the virtues of love, unity and universal peace would have done some good to the raw minds of terrorists and in turn impacted the larger world in several positive ways.